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Explain this step on the proof of the curvature of the vector $r(t)$?

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Which part you don't understand? –  Jack Mar 22 '12 at 6:21
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Differentiate $T\times T=0$ and you are done –  Blah Mar 22 '12 at 6:47
    
I don't get it... –  Dokkat Mar 24 '12 at 7:26
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